New pipelines are beginning to carry a glut of domestic crude from the middle of the country to Texas' Gulf Coast, boosting the fortunes of the area's big refineries and further fueling a decline in oil imports.

Magellan Midstream Partners' Longhorn pipeline began shipping oil from West Texas to Houston in April—the first of at least seven pipeline projects that could send as much as two million barrels a day from oil-saturated choke points in Oklahoma and the interior of Texas to the largest concentration of refineries...